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.Dd April 19, 2019
.Dt CASU 9
.Os
.Sh NAME
.Nm casueword ,
.Nm casueword32 ,
.Nm casuword ,
.Nm casuword32
.Nd fetch, compare and store data from user-space
.Sh SYNOPSIS
.In sys/types.h
.In sys/systm.h
.Ft int
.Fo casueword
.Fa "volatile u_long *base"
.Fa "u_long oldval"
.Fa "u_long *oldvalp"
.Fa "u_long newval"
.Fc
.Ft int
.Fo casueword32
.Fa "volatile uint32_t *base"
.Fa "uint32_t oldval"
.Fa "uint32_t *oldvalp"
.Fa "uint32_t newval"
.Fc
.Ft u_long
.Fo casuword
.Fa "volatile u_long *base"
.Fa "u_long oldval"
.Fa "u_long newval"
.Fc
.Ft uint32_t
.Fo casuword32
.Fa "volatile uint32_t *base"
.Fa "uint32_t oldval"
.Fa "uint32_t newval"
.Fc
.Sh DESCRIPTION
The
.Nm
functions are designed to perform atomic compare-and-swap operation on
the value in the usermode memory of the current process.
.Pp
The
.Nm
routines reads the value from user memory with address
.Pa base ,
and compare the value read with
.Pa oldval .
If the values are equal,
.Pa newval
is written to the
.Pa *base .
In case of
.Fn casueword32
and
.Fn casueword ,
old value is stored into the (kernel-mode) variable pointed by
.Pa *oldvalp .
The userspace value must be naturally aligned.
.Pp
The callers of
.Fn casuword
and
.Fn casuword32
functions cannot distinguish between -1 read from
userspace and function failure.
.Sh RETURN VALUES
The
.Fn casuword
and
.Fn casuword32
functions return the data fetched or -1 on failure.
The
.Fn casueword
and
.Fn casueword32
functions return 0 on success, -1 on failure to access memory,
and 1 when comparison or store failed.
The store can fail on load-linked/store-conditional architectures.
.Sh SEE ALSO
.Xr atomic 9 ,
.Xr fetch 9 ,
.Xr store 9
